Switching to MVVM

using System;
using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Controls;

namespace Step3
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public static DependencyProperty PrincipleProperty =
            DependencyProperty.Register("Principle", typeof(double), typeof(MainWindow));

        public double Principle
        {
            get { return (double)GetValue(PrincipleProperty); }
            set { SetValue(PrincipleProperty, value); }
        }

        public static DependencyProperty InterestRateProperty =
            DependencyProperty.Register("InterestRate", typeof(double), typeof(MainWindow));

        public double InterestRate
        {
            get { return (double)GetValue(InterestRateProperty); }
            set { SetValue(InterestRateProperty, value); }
        }

        public static DependencyProperty DurationProperty =
            DependencyProperty.Register("Duration", typeof(int), typeof(MainWindow));

        public int Duration
        {
            get { return (int)GetValue(DurationProperty); }
            set { SetValue(DurationProperty, value); }
        }

        public static DependencyProperty PaymentProperty =
            DependencyProperty.Register("Payment", typeof(double), typeof(MainWindow));

        public double Payment
        {
            get { return (double)GetValue(PaymentProperty); }
            set { SetValue(PaymentProperty, value); }
        }

        private ObservableCollection<PaymentInfo> payments = new ObservableCollection<PaymentInfo>();

        public MainWindow()
        {
            InitializeComponent();

            DataContext = this;
        }

        private void btnExit_Click(object sender, RoutedEventArgs e)
        {
            Close();
        }

        private void btnCalculate_Click(object sender, RoutedEventArgs e)
        {
            CalculatePayment();
        }

        // Calculate the complete amortization schedule and fill the list control
        private void CalculatePayment()
        {
            int totalpayments = Duration * 12;

            Title = "Amortization Schedule for " +
                Convert.ToString(totalpayments) + " Payments";

            double monthlyInterest = CalculateMonthlyInterest(InterestRate);

            // calculate interest term
            double interestTerm = Math.Pow((1 + monthlyInterest), totalpayments);

            // calculate payment
            Payment = (Principle * monthlyInterest) / (1 - (1 / interestTerm));

            payments.Clear();

            for (int iIndex = 1; iIndex <= totalpayments; ++iIndex)
            {
                PaymentInfo paymentInfo = new PaymentInfo();
                paymentInfo.PaymentNo = iIndex;
                paymentInfo.Balance = CalculateBalance(iIndex);
                paymentInfo.Payment = Payment;
                paymentInfo.Interest = CalculateInterestPart(iIndex);
                paymentInfo.Principle = CalculatePrinciple(iIndex);

                payments.Add(paymentInfo);
            }

            lstAmortization.ItemsSource = payments;
        }

        // Calculate the remaining balance at particular payment
        private double CalculateBalance(int month)
        {
            double monthlyInterest = CalculateMonthlyInterest(InterestRate);

            double interestTerm = Math.Pow((1 + monthlyInterest), month);
            double totalInterest = Principle * interestTerm;
            double totalPaid = Payment * (interestTerm - 1) / monthlyInterest;
            return totalInterest - totalPaid;
        }

        // Calculate the Interest part of any particular payment
        private double CalculateInterestPart(int month)
        {
            double monthlyInterest = CalculateMonthlyInterest(InterestRate);

            double interestTerm = Math.Pow((1 + monthlyInterest), (month - 1));
            double totalInterest = Principle * interestTerm;
            double totalPaid = Payment * (interestTerm - 1) / monthlyInterest;
            return (totalInterest - totalPaid) * monthlyInterest;
        }

        // Calculate the principle part of any particular payment
        private double CalculatePrinciple(int month)
        {
            return Payment - CalculateInterestPart(month);
        }

        // Calculate the monthly interest rate
        private double CalculateMonthlyInterest(double InterestRate)
        {
            double monthlyInterest = InterestRate;
            monthlyInterest /= 100;
            monthlyInterest /= 12;

            return monthlyInterest;
        }
    }

    public class PaymentInfo
    {
        public int PaymentNo
        { get; set; }

        public double Payment
        { get; set; }

        public double Principle
        { get; set; }

        public double Interest
        { get; set; }

        public double Balance
        { get; set; }
    }

    public class MyValidation : ValidationRule
    {
        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            int number = Int32.Parse((string)value);

            if (number < 0)
            {
                return new ValidationResult(false, "Please enter values greater than zero.");
            }

            return new ValidationResult(true, null);
        }
    }
}